A relative vector (free vector) in 3D space. The triple is the x, y, z components,
and the last item is the squared magnitude of the vector, which is stored
with it to speed up various operations. It is suggested you use makeRel3
to create one of these, unless the magnitude is easily apparent, e.g. Rel3
(0, 1, 1) 2

A line in 3D space. A line is a point and a free vector indicating
direction. A line may be treated by a function as either finite (taking
the magnitude of the free vector as the length) or infinite (ignoring the
magnitude of the direction vector).